For a discussion of the BOD (Biochemical Oxygen Demand), see the following articles: E. M. Davis, BOD vs. COD vs. TOC vs. TOD, Water and Wastes Engineering, Feb. 1971, pages 32-35; K. M. Peil and A. F. Gaudy, Jr., Kinetic Constants for Aerobic Growth of Microbial Populations Selected with Various Single Compounds and with Municipal Wastes as Substrates, Vol. 21, No. 2, Feb. 1971, pages 253-256; E. L. Barnhart and G. J. Kehrberger, A Critical Review of UOD Analysis Using Respirometer Techniques, Vol. 135, Part 2, Engineering Bulletin of Purdue University, 1969, pages 1385-1397; Sludge/BOD Ratio is key to Wastewater Cleanup, Chemical Engineering, Apr. 21, 1969, pages 60-62.
The object of any waste water treatment operation is to remove the contaminants from the waste water so that the effluent, or treated water, has a low enough BOD level that it can be lawfully discharged into the nearest water source.
The activated sludge process is not a single precise biological or physical process. It is the combination of several basic processes interacting both independently and dependently. The activated sludge process constitutes four basic sub-processes as follows: (1) Basic respiration and metabolism of aerobic bacterial cells. (2) Mass transfer of nutrients in the substrate and dissolved oxygen to the cell surface and removal of metabolic waste products. Nutrients, commonly referred to as BOD, are the carbonaceous organic compounds in the liquid medium that provide food for cell growth and energy. (3) Supplying of oxygen from the atmosphere or other sources to the waste in the liquid. (4) The aggregation or flocculation of the individual microorganisms into masses or matrices usually of the order of a hundred times the size of the individual bacterial cells.
Prior to the present invention, it was thought that the greatest influence which the engineer could exercise over the waste treatment process in obtaining the optimum performance from the activated sludge process was in the second listed basic sub-process. Great importance was placed on the influence which the type and degree of agitation or turublence has on the transfer of DO and thought to be essential for mass transfer of nutrients to individual cells and into and through agglomerations of these cells. See the following articles concerned with the importance of the dissolved oxygen: R. A. Ryder, Dissolved Oxygen Control in Activated Sludge, Vol. 135, Part 1, 1969, Engineering Bulletin of Purdue University, pages 238-253; G. F. Bennett, Relationship Between Dissolved Oxygen Concentration and Oxygen Utilization Rates in Activated Sludge Waste Treatment, Vol. 63, No. 78, Chemical Engineering Progress Symposium Series, pages 171-177; G. J. Thabaraj and G. J. Gaudy, Jr., Effect of Dissolved Oxygen Concentration on the Metabolic Response of Completely Mixed Activated Sludge, Vol. 135, Part 1, 1969, Engineering Bulletin of Purdue University, pages 69-86.
It was thought that the nutrient or organic waste substrate which is the source of energy and growth material for the sludge cells could not be adequately absorbed and adsorbed onto the cells, when the waste water and sludge were mixed together, unless sufficient microagitation or turbulence was provided, and unless the proper amount of dissolved oxygen in the waste water was also provided (see an article by A. A. Kalinske, Journal of Water Pollution Control Federation, Vol. 43, No. 1, January 1971, Effect of Dissolved Oxygen and Substrate Concentration on the Uptake Rate of Microbial Suspensions).
In the conventional activated sludge system, activated sludge (microorganisms that are viable or "hungry" and require soluble organic food) is mixed with raw sewage together with large amounts of air or oxygen. Within a short time, the hungry organisms absorb and adsorb most of the soluble organic waste. In the case of air, as in conventional systems, the air serves to agitate the sludge and keep it in suspension to provide adequate distribution of organisms in the sewage; but, in systems employing relatively pure oxygen, it is wasteful and uneconomical if used for agitation. Some systems that make use of oxygen for agitation in the early stages of activated sludge operation are completely enclosed in order to conserve the unassimilated oxygen which is used in a later stage downstream when the organisms have absorbed large quantities of organic matter. In the end, these systems waste significant quantities of oxygen to the atmosphere.